In order to improve various characteristics of industrial products such as cutting tools, wear-resistant tools, molds, and electronic components, a coating having characteristic physical properties has conventionally been provided on a surface of such a substrate through vapor deposition.
Vapor deposition includes physical vapor deposition (PVD) or chemical vapor deposition (CVD). In particular, since a coating formed through PVD decreases the likelihood of a deterioration in strength of the substrate, it has widely been used for a cutting tool required to be high in strength, such as drills, end mills, and indexable inserts for milling, wear-resistant tools, and molds. In particular, demands for tools include adaptation to dry cutting without the use of a cutting fluid and higher cutting speed. In order to meet such demands, the coating provided on the surface of the substrate is required to have high hardness and good wear resistance.
For the purpose of improving the hardness and wear resistance of a coating, a coating in which a specific compound is stacked or combined on a scale as fine as a nanometer, thus having nanometer order, has been studied. For example, PTD 1 (Japanese Patent Laying-Open No. 07-003432) discloses a coating in which two or more types of layers each having a thickness on the order of nanometers are alternately stacked, and PTD 2 (Japanese Patent Laying-Open No. 06-279990, PTD 3 (Japanese Patent Laying-Open No. 2001-293601), and PTD 4 (Japanese Patent Laying-Open No. 2002-096205) each disclose a coating having a layer containing fine particles on the order of nanometers.
In such a coating which is a stack or combination on a scale as small as the nanometer order, a hardness of the coating itself tends to be high owing to nanometer size effect. Furthermore, strain energy is likely to be accumulated at an interface between compounds different in composition. Coatings in which strain energy is accumulated have been known to be high in hardness. That is, according to the conventional art, attempts to improve the hardness of a coating based on the nanometer size effect and accumulation of strain energy and to thereby improve wear resistance of the coating have been made.